1. Field of the Invention
The present invention relates to an optical pickup apparatus for recording and reproducing information on and from an optical disk and a technique which is advantageous when used in, for example, an optical pickup apparatus adapted to optical disks having recording surfaces forming a plurality of layers.
2. Description of the Related Art
For example, in a case wherein information is recorded and reproduced on and from an optical disk having recording surfaces forming two layers, laser light travels different distances before it reaches a recording surface after passing through a transparent layer of the optical disk when a recording surface on the first layer is accessed and when a recording surface on the second layer is accessed.
Further, when laser light enters the transparent layer and converges on the recording surface, the laser light undergoes spherical aberration as it passes through the transparent layer. Spherical aberrations having different magnitudes occur when the recording surface on the first layer is accessed and when the recording surface on the second layer is accessed because the light travels different distances in passing through the transparent layer. In particular, in an apparatus that accommodates writable DVDs (digital versatile disks) and optical disks adapted for violet lasers, a lens having a great numerical aperture must be used as an objective lens for converging laser light on a recording surface. Since the great numerical aperture results in a spherical aberration having a great magnitude, a difference between spherical aberrations on the recording surface of the first layer and the recording surface of the second layer cannot be ignored.
Under the circumstance, a spherical aberration has been corrected in an optical pickup apparatus adapted to optical disks as described above by providing an expander lens comprising two lenses on an optical path from a laser diode to an objective lens and by dynamically changing the interval between the lenses to change the beam diameter.
Conventional configurations for changing the lens interval of the expander lens of such an optical pickup apparatus include a configuration in which a lens holder is supported such that it can be axially slid; a threaded hole is provided in the lens holder and engaged with a screw shaft; and the lens holder is translated by rotating the screw shaft with a motor to change the lens interval.
As disclosed in JP-A-2003-30890, JP-A-2003-45067 and JP-A-2003-115127, configurations have been proposed, in which either or both of lenses constituting an expander lens are movably supported with springs and in which an electromagnetic force is applied to lens holders using a coil and a magnet to change the lens interval.
However, in the above-described driving configuration using a screw shaft, since a clearance between the screw shaft and the threaded hole results in a positional deviation of the lens holder attributable to so-called backlash even if the screw shaft is stopped at a predetermined position of rotation, the lens positions can be changed even by a slight vibration and are therefore difficult to adjust accurately. Further, this configuration has a problem in that it is difficult to achieve compactness because a motor is required and in that it is difficult to change the position of the lens holder quickly because the driving force is generated by the screw shaft.
The diving configurations disclosed in JP-A-2003-30890, JP-A-2003-45067 and JP-A-2003-115127 have a problem in that the lenses can not be maintained at adjusted positions unless a current is continuously passed through the coil that applies the driving force to the lens holder.